Chemistry

Nadirov R.K.¹, Aitkulova R.E.², Nadirov K.S.²

¹Kazakh National University, Almaty, Kazakhstan

²South Kazakhstan State University, Shymkent, Kazakhstan

THEORETICAL COMPUTATION OF ELECTRODE POTENTIAL OF ELECTROHEMICAL OXIDATION OF EUPAFOLIN

The oxidation of eupafolin (1) in aqueous-acetonitrile solution involves a two-electron process, which is represented below:

-2Н^+ -2е (1)

(1) (2)

Alternatively, eupafolin can be converted to its oxidized form (2) using o-benzoquinone (3) as a reference molecule according to the following isodesmic reaction [1]:

(1)

+

(3)

(2) +

(2) (4)

The difference between the electrode potential of two species can be obtained from the change in Gibbs free energy of reaction (2) [2]:

(3)

where n is number of electrons transferred (n = 2 in this case) and F is the Faraday constant.. In order to obtain standard electrode potential of eupafolin, the standard

change of Gibbs free energy of reaction (2), ΔG⁰, is required along with the experimental value of electrode potential of the reference molecule, ortho benzoquinone [3]. In order to calculate the standard Gibbs energy of reaction (2), ΔG⁰, one should calculate the standard Gibbs energy of each component, , in reaction (2) and this is possible by calculation of the gas-phase energy of each component, , together with the solvation energy of the component, [2]:

(4)

(5)

Thus, Gibbs energy of each molecule in the gas phase is necessary for the calculation of electrode potential of eupafolin.

According with [2], in the present work, the gas phase contribution to the Gibbs energy, , was determined from DFT calculations [3].

The entropies have been used to convert the internal energies to the Gibbs energies at 298 K. We have used polarizable continuum model (PCM) of solvation [5] in order to calculate solvation energies, . HYPERCHEM (7.0) software have been employed for all DFT calculations.

Table shows the calculated Gibbs energy of molecules for both reduced and oxidized forms in the gas phase. Solvation energies are also computed in order to convert gas-phase energies to energies in solution phase. The total Gibbs free energy of each component in the presence of solvent are also included in Table 1.

Using these values together with the standard Gibbs free energy of pyrocatechol (4) which are presented in Table, and employing Eqs. (3) and (5), the standard electrode potential of eupafolin is calculated to be 0.724 V.

Table – Heat of formation, entropies and Gibbs free energies for oxidized (2) and reduced (1) forms of eupafolin and also o-benzoquinone (3) and pyrocatechol (4)

	Heat of formation, kj/mol (gas fase)	Entropies, kj/mol•K (gas phase)	Gibbs free energies, kj/mol
			Gas phase	Aqueus phase
(1)	-851.473	0.336	-951.601	-951.719
(2)	-665.494	0.317	-759.960	-760.020
(3)	-95.187	0.117	-130.053	-130.362
(4)	-263.223	0.098	-292.427	-292.517

All energies are at 298 K

References

1. B.A. Bohm, Introduction to Flavonoids, Harwood Academic Publishers, Singapore, 1998 (Chapter 2).

2. Hamid R. Zare, Mansoor Namazian, Navid Nasirizadeh, J. Electroanal. Chem 584 (2005) 77.

3. M. Namazian, P. Norouzi, J. Electroanal. Chem. 573 (2004) 49.

4. A.D. Becke, J. Chem. Phys. 98 (1993) 5648.

5. S. Miertus, E. Scrocco, J. Tomasi, Chem. Phys. 55 (1981) 117.